Proton NMR spectroscopy is being used to study the interaction of a soluble heme enzyme (Cytochrome c Peroxidase) with its physiological redox partner, yeast iso-1 cytochrome c. In addition, comparison studies with functional, but nonphysiological redox partner, cytochromes c from tuna and horse are being carried out. In low salt solutions the peroxidase spontaneously forms noncovalent complexes with each of the cytochromes c. The structures of the individual proteins is being explored by NMR, as are the dynamics and thermodynamics of the formation of the individual noncovalent redox complexes. The Cytochrome c Peroxidase:Yeast iso-1 cytochrome c complex occurs naturally in yeast mitochondria. So far dynamic NMR studies have documented the complex-induced spectral changes associated with 1:1 complex formation, allowed quanitization of the lifetimes of the complexes in solution at low ionic strength, determined the stoichiometry of the primary complex in solution, measured the equilibrium constants for complex formation as a function of ionic strength between 10 mM and 600 mM, and shown that the dynamic exchange of the horse and tuna ferricytochrome c out of the 1:1 complex is quantitatively faster than for the yeast iso-1 ferricytochrome c complex. The goal of this project with respect to use of NMRFAM is as follows. 1. Take advantage of the multiple fields available at one site to define the field-dependent usefulness of specific NMR experiments applied to paramagnetic heme proteins of different sizes. The complicating factor in this is the Curie-spin relaxation and Relaxation Allowed Coherence Transfer. 2. Complete the 3-dimensional solution structure of yeast iso-1 ferricytochrome c (diamagnetic) in solutions maintained at two different ionic strengths (10 mM and 250 mM salt).